US counter-terrorism officials are chasing a credible but unconfirmed al Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington.

It was the first word of an "active plot" timed to coincide with the sombre commemoration of the terror group's September 11 attacks a decade ago that killed nearly 3,000 people.

Counter-terrorism officials were investigating the threat as police in New York and Washington said they would increase their already stepped-up staffing levels in light of the recent intelligence.

Law enforcement officials were pursuing three people who may be travelling to the US or who have recently entered the country, based on the detailed information received by the US intelligence community late on Wednesday, officials said.

The intelligence suggested that al Qaida planned to car bomb one of the two cities that were hit 10 years ago.

Vice President Joe Biden said that there was no confirmation that anyone had travelled into the US for such a plot although the tip came from a credible source. "There's no certitude," he said. "The thing we are all most worried about is what they call a 'lone ranger,' a lone actor, not some extremely complicated plan like it took to take down the World Trade towers," said Mr Biden.

The threat came in a single piece of information and was so specific - and came at such a time of already heightened alert - that it could not be ignored, officials said.

A US official said the source of the terror tip indicated that al Qaida's new leader, Ayman al-Zawahri, was involved in planning the plot. But the official also said that many in the intelligence community question that and other aspects of the source's information.

Several hundred military and civilian workers gathered at the Pentagon for a sombre 9/11 ceremony in the courtyard, near where Flight 77 slammed into the building nearly 10 years ago.

US defence secretary Leon Panetta told the crowd that violent extremism remains a deadly threat, and "we are dealing with their threats as we speak". Mr Panetta offered no details on the latest threat. But he said the operation that took out Osama bin Laden made "a very clear point to our country and to the world: that nobody attacks this country and gets away with it".